The invention relates to a process for preparing diaryl carbonates and/or alkyl aryl carbonates from dialkyl carbonates and aromatic hydroxy compounds using a reactive dividing wall column.
The preparation of aromatic and aliphatic-aromatic carbonic esters (carbonates) by transesterification starting from aliphatic carbonic esters and aromatic hydroxy compounds is known in principle. This is an equilibrium reaction in which the position of the equilibrium is shifted virtually completely in the direction of the aliphatically substituted carbonates. It is therefore relatively easy to prepare aliphatic carbonates from aromatic carbonates and alcohols. However, to carry out the reaction in the converse sense in the direction of aromatic carbonates, it is necessary to shift the very unfavourable equilibrium effectively to the side of the aromatic carbonates, for which not only very active catalysts but also suitable process conditions have to be employed.
It is known that such equilibrium reactions can be carried out in columns and can in this way be shifted advantageously in the direction of formation of the desired product, as described, for example, by U. Block, Chem.-Ing. Techn. 49, 151 (1977), DE-A 38 09 417, B. Schleper, B. Gutsche, J. Wnuck and L. Jeromin, Chem.-Ing.-Techn. 62, 226 (1990), Ullmans Encyclopädie der technischen Chemie, 4th Edition, Vol. 3; p. 375 ff. 1973).
In the known processes, the transesterification is therefore preferably carried out continuously in a countercurrent transesterification in one or more reaction columns.
However, the processes known from the literature (e.g. EP 0 461 274, DE-A 42 26 755, DE-A 42 26 756) usually describe only those process steps in which the reaction to form diaryl carbonate by transesterification and/or disproportionation takes place. Current patent applications, for example WO 2006/033291 A1, EP 1 775 280A1, EP 1 767 516 A1, EP 1 767 517 A1, EP 1767 518 A1, EP 1 762 559 A1 and EP 1 762 560 A1, also give information regarding the physical designs of reaction columns for preparing diaryl carbonates. The mixtures of reaction alcohol and dialkyl carbonate which are formed in this process are separated in apparatuses separate from the reaction apparatuses.
To improve the economics of the process compared to the prior art, both approaches for reducing the energy consumption and measures for reducing the capital costs are required. On the basis of experience, the two requirements can be fulfilled only with restrictions in industrial practice.
EP-A 0 461 274 describes a continuous transesterification process for preparing aromatic carbonates in one or more multistage columns connected in series, with dialkyl carbonates or alkyl aryl carbonates being reacted with phenols and the volatile products, namely the reaction alcohols and dialkyl carbonates being taken off at the top of the columns and the high-boiling products such as diaryl carbonates being taken off at the bottom of the columns. Since both dialkyl carbonates and the reaction alcohols are taken off at the top of the column, at least one further step is required to separate these components.
DE-A 42 26 756 describes a two-stage process for preparing diaryl carbonates by transesterification of a dialkyl carbonate by means of an aromatic hydroxy compound, in which the corresponding alkyl aryl carbonate is firstly formed from the starting materials in a first stage and the diaryl carbonate is formed in a second stage. The information given in the process description is restricted to the reaction conditions, the catalyst used and the design of the reaction columns. In the case of this patent application, too, reaction alcohol and dialkyl carbonate are taken off at the top of the columns.
DE-A 42 26 755 describes a process for preparing diaryl carbonates in two reaction columns which are coupled with one another in terms of energy and material, in which an aromatic hydroxy compound and a dialkyl carbonate are reacted in the first stage and the alkyl aryl carbonate formed is converted into diaryl carbonate in the second stage either by transesterification with the aromatic hydroxy compound or by disproportionation. However, this has the problem that the integration of the process in terms of material and energy does not allow the reaction conditions to be selected optimally for the formation of the alkyl aryl carbonate or diaryl carbonate since these are fixed by the virtually identical pressures prevailing in the two steps.
EP-A 781 760 describes a continuous process for preparing aromatic carbonates by reaction of a dialkyl carbonate with an aromatic hydroxy compound in the presence of a catalyst, continuous removal of the aromatic carbonate formed in the reaction, the alcoholic by-products, the dialkyl carbonate and the aromatic hydroxy compound, with the dialkyl carbonate and the aromatic hydroxy compound being recirculated to the reaction. In this process, too, only two fractions which are taken off at the upper end and at the bottom, respectively, of the reactor are obtained in the reaction part of the process.
WO-A 2006/001256 describes a process in which an aromatic hydroxy compound is reacted with a dialkyl carbonate in the presence of a catalyst and also an industrial apparatus suitable for this purpose. Here too, the fractions are taken off only as overhead product or bottom product.
Without appropriately efficient integration into terms of apparatus and energy, the capital and energy costs of the above-described processes are, as is known, high, which in turn makes the advantages of the phosgene-free preparation of aryl carbonates questionable from an ecological and economic point of view.
Approaches for integration in terms of apparatus and energy comprise the use of dividing wall columns.
An example of the mode of operation of dividing wall columns in reactive separation processes is described in EP 0 126 288B1. However, no information as to how this technology can be applied in the preparation of diaryl carbonates by transesterification of a dialkyl carbonate is given here.
A further example of a reactive separation process may be found in Ind. Eng. Chem. Res. 2007, 46, 3709-3719. This disclosure describes a transesterification of dimethyl carbonate with ethanol to form the aliphatic diethyl carbonate. Thus, no diaryl carbonate is produced in this process either.
In the transesterification of an aromatic hydroxy compound with an alkyl carbonate, the latter is generally employed in excess in order to achieve a high conversion based on the aromatic hydroxy compound.
In the case of a transesterification in a reaction column, both the dialkyl carbonate used in excess and the reaction alcohol formed in the reaction are separated off at the top of the column. The aromatic hydroxy compound, the alkyl aryl carbonate formed in the reaction, possibly diaryl carbonate and dialkyl carbonate are preferably present in the bottom product. In the case of a homogeneously catalyzed reaction, the bottom product also contains the catalyst.
A disadvantage of this mode of operation is that the dialkyl carbonate used in excess has to be separated from the reaction alcohol in a separate step.
Although a separation of reaction alcohol from the dialkyl carbonate in an enrichment section above the reaction zone is possible in principle, it is relatively ineffective in terms of the conversion since it is thus only introduced from the top into the reaction zone. In this way, only the separation of dialkyl carbonate and aromatic hydroxy compound is increased. However, this effect should be avoided in order to be able to ensure a high conversion of aromatic hydroxy compound.
There is therefore a continuing need to provide a process for preparing aromatic carbonates, i.e. diaryl carbonates and/or alkyl aryl carbonates, preferably diaryl carbonates, which does not have the abovementioned disadvantages and in which a process which is integrated in terms of energy and apparatus compared to the abovementioned known processes is provided.
The object of the invention was accordingly to provide a process for preparing aromatic carbonates, i.e. diaryl carbonates and/or alkyl aryl carbonates, in which, compared to known processes, the reaction of the dialkyl carbonate with the aromatic hydroxy compound and the separation of the mixture of reaction alcohol and dialkyl carbonate by distillation are coupled in terms of apparatus and energy.